This invention relates to wireless digital telephony. In one aspect, it provides a single bit bandpass analog-to-digital converter for extracting lower frequency digital signals modulating a higher frequency carrier. In another aspect, it provides a component insensitive, analog bandpass filter. This filter is not only a preferred embodiment of one of the elements of the single bit bandpass analog-to-digital converter, but also is useful in purely analog applications, especially those in which the passband must be precisely centered on a specific frequency.
In digital telephony, the radio frequency RF modulated by the digital signal is very high, perhaps 900 MHz. As shown in FIG. 1, this modulated signal is passed through a low noise amplifier LNA and fed to a first mixer M1. The first mixer M1 also receives a signal from a first local oscillator LO1, which differs in frequency from RF by a first intermediate frequency IF1. If IF1 is 100 MHz, as is typical, then LO1 has a frequency of either 800 MHz or 1 GHz. In either event M1 produces an IF1 output, still modulated by the digital signal.
The IF1 signal is passed through a first bandpass filter BPF1, amplified by a first intermediate frequency amplifier IFA1, and mixed in a second mixer M2 with the output of a second local oscillator LO2. IFA1 must have good fidelity, but not the extreme fidelity of LNA; this is the reason for the two amplification stages. LO2 has a frequency which differs from IF1 by a second intermediate frequency IF2. If IF2 is 1.8 MHz, as is typical, then LO2 has a frequency of either 101.8 MHz or 98.2 MHz. In either event M2 produces an IF2 output, still modulated by the digital signal. This is amplified by a second intermediate frequency amplifier IFA2.
Conventional processing at this stage is shown in FIG. 2. The signal from IFA2 is fed to both a third mixer M3 and to a fourth mixer M4. A third local oscillator LO3, also producing a signal at IF2, is fed to both mixers, thereby demodulating the digital signal entirely. In the case of M4, LO3's signal first passes through a 90 degree phase delay apparatus PDA. The output of M3 is therefore a demodulated inphase signal DI, and the output of M4 is a demodulated quadrature signal DQ. These are respectively digitized by an inphase analog-to-digital converter IADC and a quadrature analog-to-digital converter QADC.
As shown in FIG. 3, LO3, M3, PDA and M4 can be eliminated and the signal from IFA2 can be fed directly to an intermediate frequency analog-to-digital converter IFADC. This is often desired, since LO3, M3, PDA, and M4 are expensive and cumbersome. Inphase and quadrature demodulation then takes place in the digital domain.
Demodulated signals DI and DQ need to have as broad a bandwidth as the human ear, but no broader. 15 KHz is ample. Even allowing for negative frequencies, a band from -15 KHz to +15 KHz is only 30 KHz wide.
This presents a design opportunity. The conventional IFADC of FIG. 3 has good fidelity from 0 Hz all the way to the top of the passband, 1,815 MHz, even though fidelity is required only for the passband itself, 1,785 MHz to 1,815 MHz. If the noise which inevitably attends the digitization process can be excluded from this narrow passband, it makes no difference that the noise outside this passband is retained, or even increases. The prior art, in keeping noise out of the passband, was forced to keep it out of all lower frequencies as well. This was not cost-effective, and presented problems of size, weight, and power consumption as well.
The bandpass analog-to-digital converter contemplated for doing this can have an output of as little as a single bit. Such a converter can very conveniently be mechanized using a component insensitive analog bandpass filter. Such a filter would have applications far removed from digital telephony. It could be used even in purely analog applications, provided that the passband was required to be precisely centered on a digital frequency.